(1) Field of the Invention
The present invention relates to a process for the preparation of a high-molecular weight polyester which is valuable as a starting material of fibers, films and other shaped articles having excellent physical properties. More particularly, the present invention relates to a process for preparing a high-molecular weight polyester having an intrinsic viscosity of at least 1.0 by melt polymerization.
(2) Description of the Related Art
It is known that polyesters represented by polyethylene terephthalate can be used as the starting materials for the production of fibers, films and other shaped articles.
The molecular weight is an important factor having influences on the strength and other physical properties of these shaped articles, and increase of the molecular weight has been desired for improving these physical properties.
However, it has been considered difficult to obtain high-molecular weight polyesters according to the ordinary melt polymerization including the glycol-removing polycondensation customarily adopted, because (1) the glycol-removing polycondensation is an equilibrium reaction and hence, the molecular weight cannot be elevated beyond a level corresponding to the partial pressure of the glycol and (2) since the polycondensation reaction is carried out at a high temperature, also a decomposition reaction is caused.
As means for solving these problems, there is ordinarily adopted a solid phase polymerization method in which the polymerization reaction is carried out at a low temperature. However, the solid phase polymerization method is defective in that a long time is required for completion of the reaction and the obtained polymer is hardly soluble. Moreover, the intrinsic viscosity of the polymer obtained according to the solid phase polymerization method is about 1.4 at highest.
Various trials have been made to improve the melt polymerization method for obtaining high-molecular weight polyesters. As a typical instance, there can be mentioned a method using a chain extender. However, this method is disadvantageous in that an expensive chain extender should be used, portions connected through the chain extender are thermally unstable, a compound formed as a by-product from the chain extender is left in the polymer or incorporated in ethylene glycol formed as a by-product and recovery of ethylene glycol is rendered difficult. Furthermore, even if this method is adopted, it is difficult to obtain a high-molecular weight polyester having an intrinsic viscosity of at least 1.8.